In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium to form characters or images thereon. The ink-jet printing methods have become rapidly spread because of various advantages such as easiness of full coloration, low cost, capability of using a plain paper as the recording medium, non-contact with printed characters or images, etc.
In recent years, in order to impart good weathering resistance and water resistance to printed matters, an ink that contains a pigment as a colorant has been extensively used. In view of attaining good ejection stability and storage stability of the ink, in order to stably compound the pigment in the ink, the technologies for encapsulating the pigment with a polymer using a dispersant capable of modifying a surface of the pigment, etc., have been developed.
JP 9-272831A (Patent Literature 1) aims at providing a recording medium liquid that is excellent in long-term storage stability of pigment components, density and clarity of images formed, etc., and discloses an recording medium liquid for ink-jet printing which contains a pigment composite polymer obtained by heating a polymer including a segment (A) containing a reactive group capable of reacting with a functional group present on a surface of the pigment and a segment containing no reactive group and having a higher affinity to a liquid medium than that of the segment (A), together with the pigment.
JP 11-343439A (Patent Literature 2) aims at providing an ink that is capable of forming images having high image density and excellent rub fastness, etc., as well as is excellent in ink-jet ejection stability, and discloses an ink including a pigment and a resin encapsulating the pigment in which the pigment is a self-dispersible carbon black containing at least one hydrophilic group bonded to a surface thereof.
JP 2010-13525A (Patent Literature 3) aims at providing a water-based ink for ink-jet printing which is excellent in optical density, storage stability, etc., and discloses a water dispersion for ink-jet printing which includes a surface-treated carbon black containing an organic group having a constitutional unit constituted of an alkylene glycol on a surface thereof.